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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, my friend is having problems with his DRSSTC. The coil data is here, except the measured resonant frequency of secondary is 67 kHz : The driver is the DRSSTC1 by Steve Ward: It's a H-bridge of IGBT bricks. The problem is that with a primary peak current of 200 Amps and tuned primary, he can only get 8 cm sparks to ground and no streamers. The voltage on bridge was 180 Vdc. The primary current looks like this, 100A/div :
I am no expert on DRSSTC's but I think he should be getting much more of a spark with 200 amps. I would be grateful for any help.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
From my (modest) experience, I can say: - coil is ovrecoupled. k should be <0.2 (maybe around 0.15). He should rise the secondary coil by some cm... - Surge impedance is quite high. He should reduce primary inductance and increase primary capacitance, in order to miantain the same resonant frequency. - Probably the top load is too big and the secondary resonant frequancy is too low for such coil. He should reduce the dimension of the big topload (and re-tune the primary, obviously)
I have heard that some coil produce very modest sparks up to a certain DC bus voltage and, almost suddenly, soparks strongly increase with bus voltage. He should try to increase the bus voltage up to the maximum (350V?) and see what happen...
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I don't think that he is over coupled, you need higher coupling for higher Z. And at his resonant freq I would say that is within limits.
I would say your problem is probably tuning related by looking at the primary current from that scope shot. From the energy in the primary at the end of the burst is just kinda slowly ringing down rather than a quick ring down which indicates that most of the energy is not being transferred to the secondary.
From the sound of it, it looks like you are tuned to low, there are two solution, one crank up the bus voltage to form a streamer and have the streamer C throw the secondary circuit into tune at which point it will make a very large spark very fast. (I think this is what Luca is talking about) Or you could experiment with your tap point and tune it higher and lower to find a good operating point. I would recommend tiring this first before cracking up the bus.
This kinda thing is "almost" always tuning related with this type of coil. Don't be afraid to move your tap point whole turns at a time to get in the general neighborhood then start to fine tune 1/4 or 1/8 turns.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
That was my driver running at around 250 amps peak and i got around 8" sparks to ground when i put the secondary in. When i lowered the voltage from 80v to 40v and the current dropped to around 150a, the spark went from 8 inches to less than an inch... If i would have tried 300-350 amps primary current the sparks would be much longer than 8 inches despite the relatively small current increase.
The output depends on the current in the primary rather than the voltage on the bridge, im guessing said drsstc has an awful lot of primary impedance?
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Arcstarter wrote ...
That was my driver running at around 250 amps peak and i got around 8" sparks to ground when i put the secondary in. When i lowered the voltage from 80v to 40v and the current dropped to around 150a, the spark went from 8 inches to less than an inch... If i would have tried 300-350 amps primary current the sparks would be much longer than 8 inches despite the relatively small current increase.
The output depends on the current in the primary rather than the voltage on the bridge, im guessing said drsstc has an awful lot of primary impedance?
This is not entirely true. Yes the spark length is partly depended on the primary current, but another very big factor is how long you drive the primary for during a burst.
If you look at my project thread for my big DR you will notice it's peak primary current is rather low 800apk vs 1500Apk for a comparable system, but! I drive the primary for a much longer time 350uS vs 150uS. This is due to my higher tank impedance making the primary ring up at a slower pace. Although I drive my system different I'm still able to achieve 10+ feet with ease.
QCW and VTTC also take advantage of this type of mode, low peak currents but long burst lengths.
My point is that peak current is only half the story, you have to take into account how long you apply the current for as well. In the end the same amount of energy is still transferred to the secondary.
Registered Member #1381
Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74
Lately among hobbyists, there is this trend that started, about picking some parameters for example, the primary current vs. spark lenght... Then saying something after a few indefined "home" measurements, not seeing there are way more factors, system specs etc. worth taking into consideration.
Magnetic energy transfer in an air core resonant system isn't that simple... Im not saying that it's entirly wrong to apply some quantitative deduction to the matter. Just that most concepts start to get too simple.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Goodchild wrote ...
Arcstarter wrote ...
That was my driver running at around 250 amps peak and i got around 8" sparks to ground when i put the secondary in. When i lowered the voltage from 80v to 40v and the current dropped to around 150a, the spark went from 8 inches to less than an inch... If i would have tried 300-350 amps primary current the sparks would be much longer than 8 inches despite the relatively small current increase.
The output depends on the current in the primary rather than the voltage on the bridge, im guessing said drsstc has an awful lot of primary impedance?
This is not entirely true. Yes the spark length is partly depended on the primary current, but another very big factor is how long you drive the primary for during a burst.
If you look at my project thread for my big DR you will notice it's peak primary current is rather low 800apk vs 1500Apk for a comparable system, but! I drive the primary for a much longer time 350uS vs 150uS. This is due to my higher tank impedance making the primary ring up at a slower pace. Although I drive my system different I'm still able to achieve 10+ feet with ease.
QCW and VTTC also take advantage of this type of mode, low peak currents but long burst lengths.
My point is that peak current is only half the story, you have to take into account how long you apply the current for as well. In the end the same amount of energy is still transferred to the secondary.
Would the comparison between different coils be more accurate if we look primary RMS current instead of peak current?
Registered Member #2922
Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226
No, what they spoke is that the total transfered burst energy isn't only proportional to the current peak, or RMS current. You can get the same total energy with two differential systems:
A: Long time, like 350us and lower current peak. like 800A B: 120us and 2000A..
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DRSSTC help
